The present invention relates to a retardation film used for super twisted nematic (STN) type liquid crystal display devices, a process for producing such a retardation film, and a liquid crystal display device using said film.
Liquid crystal displays are popularly used as light-weight and thin flat panel displays for personal computers and various other display devices. Improvements of their operating performance have boosted the sales of these displays and expanded the scope of their use. The use environment of such display devices has also been widened, and it is requested that these devices be operable normally even under a high-temperature or low-temperature condition.
Among the liquid crystal display devices, the STN type is most popularly used for panel display of word processors and personal computers. In the currently available STN type liquid crystal display devices, the birefringent nature of the liquid crystal layer is compensated by a retardation film (this compensation is generally called color compensation) to produce black-and-white or color display. In the early stage of development of display devices, a liquid crystal layer has been used as compensator, but at present a uniaxially oriented polymer film is generally used.
The field of use of the STN type liquid crystal devices is also expanding and the use environment of these devices is widening. This situation calls for further improvements of operating performance and quality of display. One essential requirement is stability of the display quality with change of ambient temperature. For instance, when a display device is set and used in an automobile where the internal temperature varies greatly according to the season, the retardation (optical phase difference) of the liquid crystal cell and that of the retardation film tend to change independently with the ambient temperature to worsen the display quality.
As the temperature rises, the retardation decreases because of relaxation of molecular orientation of the liquid crystal or polymer. Generally, relaxation of orientation of low-molecular weight liquid crystal used in liquid crystal cells is greater than that of polymers used in retardation films. Change of retardation is also greater in the liquid crystal cells. Therefore, the retardation values of said two substances, which have been optimized at room temperature, deviates from the optimum range at high temperatures, causing imperfect color compensation of the liquid crystal cell to give rise to the problems such as tinting to reduce display performance.
Thus, the retardation film comprising a polymer film has not necessarily been the best for the improvement of properties of liquid crystal cells. Attempts have been made, therefore, to use liquid crystal material for retardation film.
JP-A-3-291601 discloses a retardation film produced by applying a solution of a liquid crystal polymer substantially comprising a polyester containing ortho-substituted aromatic groups on a substrate having a known alignment agent film, heat treating the formed coating film at a temperature above glass transition temperature of the liquid crystal polymer and below liquid crystal phase/isotropic phase transition temperature to effect horizontal orientation, and then cooling the coating film to a temperature below glass transition temperature of said polymer to fix orientation. There are shown in this patent the liquid crystal polymers whose glass transition temperatures are higher than the operating temperature of liquid crystal display devices.
JP-HYO (Laid-Open PCT Appln) 4-500284 discloses a method for obtaining a retardation film by using a side chain type liquid crystal polymer or elastomer having linear or cyclic main chain, and having a glass transition temperature higher than 50.degree. C., according to which the film is stretched at a temperature above the glass transition temperature of said polymer or elastomer exceeding 50.degree. C., or a magnetic or electric field is applied to said film at a temperature above said glass transition point, thereby orienting the mesogen groups, and then orientation is fixed at a temperature below said glass transition temperature.
Further, WO 92/14180 discloses an optically anisotropic body obtained by a method which comprises heating and thereby uniaxially orienting a liquid crystal polymer having siloxane, acryl or methacryl chain as backbone and also having a glass transition temperature above room temperature, preferably in a range above the upper limit of the working temperature of a liquid crystal display, and then rapidly cooling said polymer.
However, in production of the hitherto proposed retardation films using a polymeric liquid crystal material, it was necessary to orient the mesogen groups of the polymer liquid crystal by heating and then cool the liquid crystal below its glass transition temperature to fix orientation. It was also necessary to use a polymeric liquid crystal material having a glass transition temperature higher than the working temperature. Because of higher glass transition point than room temperature, high-temperature heat treatment was necessary for aligning the optical axis of the polymer liquid crystal in the film plane. Also, since the polymer liquid crystal is in a glass state in the working temperature range, it differs from the liquid crystal cell in temperature dependency of retardation, and it does not necessarily conform well to the change with temperature of retardation of the liquid crystal cell. On the other hand, in the case of the material with low glass transition temperature, it is difficult to obtain a retardation film with satisfactory mechanical strength, and also difficulties are involved in industrial treatment of the material.
Thus, a retardation film showing temperature dependency of retardation close to that of liquid crystal cell for display has been required.